Towards integrating GRACE terrestrial water storage data into the U.S. and North American Drought Monitors

NASA's Gravity Recovery and Climate Experiment (GRACE) satellites measure time variations of the Earth's gravity field enabling reliable detection of spatio-temporal variations in total terrestrial water storage (TWS), including groundwater. The U.S. and North American Drought Monitors (USDM and NADM) are two of the premier drought monitoring products available to decision-makers for assessing and minimizing drought impacts, but they rely heavily on precipitation indices and do not currently incorporate systematic observations of deep soil moisture and groundwater storage conditions. Thus GRACE has great potential to improve the Drought Monitors by filling this observational gap. This presentation will provide an update on our progress towards integrating GRACE TWS data into the USDM and NADM. Horizontal, vertical and temporal disaggregation of the coarse-resolution GRACE TWS data has been accomplished by assimilating GRACE TWS anomalies into the Catchment Land Surface Model using an ensemble Kalman smoother. The Drought Monitors combine several short-term and long-term drought indices and indicators expressed in percentiles as a reference to their historical frequency of occurrence for the location and time of year in question. To be consistent, we generated a climatology of estimated soil moisture and ground water based on a 60-year Catchment model simulation and subsequently used it to convert seven years of GRACE assimilated fields into soil moisture and groundwater percentiles, for systematic incorporation into the objective blends that constitute Drought Monitor baselines. The potential benefit of including the GRACE-based drought indicators was analyzed by comparing the soil moisture and ground water maps against the current suite of short-term and long-term objective indicators and by correlating detectable differences at the regional and local scale to the final U.S. Drought Monitor product, which incorporates subjective input from a network of local climate and water experts. Finally, we evaluated GRACE assimilating Catchment model output against independent datasets including soil moisture observations from Aqua/AMSR-E and groundwater level observations from the U.S. Geological Survey's Groundwater Climate Response Network, in order to further assess the value of incorporating GRACE assimilated fields into the Drought Monitor process.